The present invention relates generally to a heater assembly and more particularly to a self-regulating heater assembly which comprises a positive temperature coefficient heating device and is adapted for use in hostile environments.
Self-regulating heater assemblies are well known in the art. A positive temperature coefficient (PTC) heating device is a semiconductor which has an electrical resistance that is temperature sensitive. The electrical resistance of the PTC device varies proportionately with temperature. PTC devices are generally available as ceramics or polymers and are well known for use in temperature sensors, current limiters and heaters. Their usefulness as a heater is particularly attractive due to the fact that a self regulating heater can be constructed. When a current is passed through a PTC device, it produces heat by virtue of the internal resistance of the PTC device and the resultant current is similar to that of other resistance heaters except that at a certain predetermined temperature (curie point or autostabilizing temperature), the resistance begins to increase virtually exponentially, causing the power to decrease. Thus, the PTC device autostabilizes at a particular predetermined temperature. The temperature at which the PTC device autostabilizes will vary depending upon the specific PTC device. The autostabilizing temperature feature of the PTC device is useful because it can be established at a temperature which is below the ignition temperature of the heater environment or the melt point of a chemically resistant fluoropolymer coating.
PTC self-regulating heaters have not been particularly successful in the prior art when used in hostile environments such as in the chemical processing industry. In such hostile environments, strong oxidizers, free halogen ions and strong reducing acids contribute to the degradation of PTC heater assemblies.
An elongated self-regulating PTC heater assembly which includes a single heating section comprising a plurality of PTC heating elements located between a pair of electrodes is disclosed in U.S. Pat. No. 4,972,067. However, this design employs a heat shrink tube, to hold the electrodes in contact with the surfaces of the PTC heating element, inside a metallic sheath together with an outer polymeric sheath. As a result, the thickness of the plastic layers may produce sufficient thermal resistance to increase the temperature of the PTC heater assembly above its autostabilizing temperature, rendering it ineffective as a heating device. Also, this design employs only a single column of PTC elements between a pair of electrodes that run the entire length of the PTC heater assembly, thus limiting the amount of heat which can be generated.
Accordingly, it has been considered desirable to develop an improved self-regulating heater assembly which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.